1. Field of the Invention
The present invention relates to an electrophotographic apparatus, and an image processing method and an image processing program which are used for the electrophotographic apparatus. More particularly, the invention relates to an electrophotographic apparatus which more smoothes a variation of gradation of a picture of low gradation levels, thereby improving a quality of the picture, and an image processing method and an image processing program which are used for the electrophotographic apparatus.
2, Description of the Related Art
A color electrophotographic apparatus is widely used in various types of image forming apparatus, such as color printers and color copying machine. In the electrophotographic apparatus, a photoreceptor or a photosensitive member is exposed to light containing image information of a picture on an original document, and an electrostatic latent image is formed on the photoreceptor. The electrostatic latent image is developed by color toners of such colors as cyan (C), magenta (M), yellow (Y) died black (K). The color toner images are transferred and fixed to an image supporting member, e.g., a sheet of paper. In this way, the original picture is reproduced by the electrophotographic apparatus. In the laser beam printer, a laser beam is utilized for forming an electrostatic latent image on the photoreceptor. An electrostatic latent image is formed in a manner that a laser beam is irradiated on pixel areas which are arrayed in a fast or main scan direction in which the laser beam is moved for scanning and a slow scan or sub-scan direction in which the image supporting member is fed. Of those laser beam printers, a laser beam printer or the type in which the pulse width of a pulse signal for driving the laser beam is modulated, is capable of varying an irradiation area of the laser beam within the pixel area. Accordingly, even if the number or pixels per unit area is small, the laser bean printer reproduces a color picture of high resolution and good gradation.
In the laser bean printer of the pulse width modulation (PWM) type, a halftone spot half-toning technique, which is based on the multi-level dithering method, is known for one of the techniques to reproduce a gradation of a gradation image. In the multi-level dithering method, for the gradation data formed every color, which is contained in the input signal, a conversion table is referred to a conversion table, called a look-up table (LUT). The LUT contains image reproducing information determine the size and other items of a virtual dot. The size and other items (dot position in same cases) of the virtual dot within each pixel area are determined by referring to the LUT. The size of the virtual dot is expressed in terms of levels representing divisions of a size from 0 to the maximum size value of the virtual dot. By using the levels, an output signal of each pixel is multi-valued.
The term, “virtual or ideal dot”, is defined as an area within a pixel area, which is scanned with the laser beam. A size of the virtual dot as viewed in the fast scan direction is the product of multiplying a time period of driving the laser beam by a scanning speed of the laser beam. A size of it as viewed in the slow scan direction is equal to a size of the pixel area as viewed in the slow direction. The virtual dot and a “dot picture” on a final picture are different in shape for the reasons given below. Accordingly, in the specification, those different terms will be used for the distinguishing purpose. Within the virtual dot of each pixel area, the laser beam is driven and an area irradiated with the laser beam is formed on the photoreceptor. This irradiation area takes a shape radially expanded around the virtual dot since the laser beam has a size, and the rise and fall characteristics when it is driven. The irradiation area of the laser beam, serves a latent image area on the photoreceptor. The latent image in this area is developed into a visual image by toner, and the toner image is transferred onto an image supporting member, e.g., paper. As a result, a dot picture is formed in a final picture. Also during the developing process, the dot picture is further deformed from the virtual dot for some reasons. For example, the toner particles are scattered. Thus, the dot picture is deformed from the virtual dot, and its deformation depends on the electrophotography process. Therefore, the final dot picture can be controlled by controlling the virtual dot formed through the driving operation of the laser beam.
FIG. 1 is a diagram for showing a developing process in a general electrophotography process. A virtual dot area 2 on a surface of a photoreceptor or photosensitive drum 1, which is set at negative voltage (−1000V), is irradiated with a laser boom, so that an electrostatic latent image area 3 at UV is formed therein. Toner 5 charged at a negative Voltage (−500V) on a developing drum 4 is supplied to the photoreceptor drum supporting the electrostatic latent image thereon, so that the negatively charged toner particles 5 attach to the electrostatic latent image area 3 of 0V. Thereafter, the toner attaching thereto is transferred to such an image supporting member as a printing paper, so that a dot picture is formed on the image supporting member.
The halftone spot halt-toning technique forms a dot picture within a single pixel or a halftone spot consisting of a cluster of dot pictures, which covers a plurality of adjacent pixels, and reproduces a gradation of a gradation image by using the size of the halftone spot. To be more exact, as a gradation value of the gradation data of each pixel increases, a virtual dot generates, and a core of growth or a growth nucleus of the halftone spot in the final Picture is generated. When the gradation value of the gradation data further increases, the number of virtual dots and the area of them are increased, and the size of the halftone spot gradually grows. Accordingly, in the technique in which the halftone spot grows with increase of the gradation value of the input gradation data, the area of the virtual dot grows earlier in the pixel located at the center of the halftone spot (near the growth nucleus), and it grows later in the pixel closer to the periphery of the halftone spot (located apart from the growth nucleus).
FIG. 2 is a diagram exemplarily showing how a halftone spot grows. A halftone cell or halftone spot area 10 shown in FIG. 2 takes a cross shape consisting of five pixels PX1 to PX5. The center pixel PX1 serves as a growth nucleus which first grows when the gradation level of the gradation data increases. As shown in (2) in FIG. 2, when the gradation level increases, a virtual dot 12 in the center pixel PX1 gradually increases its size. When it is further increased, the size of the virtual dot 12 within the pixel PX2 increases as shown in (3) in FIG. 2. Subsequently, the virtual dot grows in the pixels PX3, PX4 and PX5 in this order, as shown in (4), (5) and (6) in FIG. 2. Finally, the virtual dot grows to fill all the pixels, and the size of the halftone spot is maximized.
Thus, those plural pixels forming one halftone spot have different area growing characteristics, respectively. For this reason, a plurality or look-up tables (LUTs) are used. In the example of FIG. 2, LUTs are assigned to the pixels PX1 to PX5, respectively. A two-dimensional array consisting of those LUTs as array elements is constructed for each halftone spot. Those two-dimensional arrays are replicatively laid over the input picture image data, as paved with tiles, whereby an overall picture is reproduced by using halftone spot 3. In the specification, the two-dimensional array will be referred to as a look-up table matrix (LUT matrix).
In the electrophotographic apparatus, the virtual dot area within the pixel is irradiated with a laser beam to form an electrostatic, latent image, and toner particles are attached to the latent image area to form a dot picture. And a halftone spot of a dot picture consisting of a plurality of pixels is made to grow. To increase a resolution of a final picture of the dot pictures, it is desirable to increase the number of screen lines connecting the halftone spots per inch as large as possible.
In connection with this, there is a proposal that in a data range of low gradation levels, to stabilize the output characteristic of the electrophotography process, a density of growing halftone spots is held to below and hence, to decrease the number of screen lines per inch. Where the number of growing halftone spots is large, the virtual dot area in each growth nucleus is too small. As a result, a print-dot missing occurs, and gradation levels are inappropriately reproduced in a final picture. Where the virtual dot area is extremely small, a critical point to determine whether or not the dot picture is formed varies depending on ambient conditions where the engine in the electrophotographic apparatus is used. Further, the print-dot missing occurs when such a critical point varies. In the data range of rather high gradation levels, all the halftone spots are simultaneously increased in size, whereby the number of screen lines is increased and hence, a sharp final picture is reproduced at high resolution.
Where such a halftone spot growing technique that in the data range of low gradation levels, the halftone spots are made to grow so as to reduce the number of screen lines, and with increase of the gradation level, the number of screen lines is increased, viz., the number of growth nucleuses of the halftone spots is increased, is employed, the gradation in the final picture, which is formed by dot pictures as finally developed, improperly increased. This fact was found by the inventor of the present patent application. More exactly, the inventor found the following fact: with increase of gradation of input picture image data, a gradation of a final picture produced after the developing process stepwise increases, and in an extreme case, the gradation of the final picture is inverted
From our further study, it was found that at a transient part where the number of screen lines is changed to another number of screen lines with increase of the gradation level of the input picture image data, the gradation of the final picture unnaturally varies or is inverted.